Iran is both today’s paramount foreign-policy challenge, and a quandary of the first order. Its nuclear program keeps expanding, its concern about international opprobrium seems limited, and nobody can be sure the United Nations Security Council will find the courage to impose more economic sanctions.

So where do we go from here? Few have thought about that challenge longer or harder than Zbigniew Brzezinski, the provocative foreign-policy icon who was White House national security adviser when the Iranian revolution erupted three decades ago and has followed the case ever since.

In an interview, Mr. Brzezinski lays out his formula. Try to stop Iran’s nuclear program, and make Tehran pay a price if it keeps pursuing it, but don’t count too much on sanctions; offer a robust American defense umbrella to protect friends in the region if Iran crosses the nuclear threshold; give rhetorical support to Iran’s opposition while accepting America’s limited ability to help it; eschew thought of a pre-emptive attack on Iran’s nuclear facilities; and keep talking to Tehran.

Above all: Play the long game, because time, demographics and generational change aren’t on the side of the current regime.

“This is a country with a growing urban middle class, a country with fairly high access to higher education, a country where women play a great role in the professions,” he says. “So it is a country which I think, basically, objectively is capable of moving the way Turkey has moved.” That is, it can evolve into a country where Islam and modernity co-exist, even if somewhat uncomfortably.